Celtic's Neil Lennon reacts as the final whistle signals his side's Champions League victory over Barcelona. Photograph: Russell Cheyne/Reuters

For a man whose name alone in Scotland is enough to inflame passion, be it furious or joyous, Neil Lennon chooses serenity when the heat of battle has subsided. This is how it was around 10pm last Wednesday, in Glasgow, when the assembled Scottish and Spanish media corps pressed in on him, eager to hear his thoughts on what had just unfolded before them all. Lennon's Celtic team, with an average age of 24 and a collective price tag of £6.5m, had inflicted defeat on Barcelona, the best team in the world and assembled at a cost of £125m. The Catalans' squad is studded with World Cup and Champions League winners, yet they were undone on the night by a Lanarkshire 18 year old who had been playing for Airdrie against teams such as East Fife and Stenhousemuir barely two years previously, before Celtic paid £50,000 for him.

Lennon was entitled to be ecstatic, triumphant even, at having steered his young charges to arguably the greatest victory witnessed at Celtic Park. Instead, the journalists encountered a reflective man, proportionate in his admiration for the way his team had played and measured in his thoughts about what the future may hold for them.

"My players are all heroes to me and I can't speak too highly of their performance," he said. "You can talk about tactics until you are blue in the face, but football is about the players. They covered themselves in glory tonight. We can give instructions to them, but it's down to the players to go and do it."

Lennon is entitled to some peace and tranquillity after 12 years of sheer tumult and brimstone. In 2002 the garlanded Ayrshire-born classical music composer, James MacMillan, penned a solo piano piece in tribute to him, called For Neil. The musician's dedication hints at the tempest that has swirled around Lennon. "I felt moved to write this little gift piece for him because of the pressures that have been put upon him, as a Celtic player, from Northern Ireland, and to remind him that he is greatly revered by many, in spite of the sectarian abuse he receives from other quarters. It is reflective and intimate, with the lilt and accent of an Irish folk song."

"The pressures that have been put upon him" began almost as soon as Lennon joined Celtic from Leicester City in 2000. From the outset, his debut at Dens Park in Dundee, he was routinely singled out for abuse at almost every ground he visited with Celtic, and most especially at Ibrox and Tynecastle, the homes of Rangers and Hearts. Part of this was due to his belligerence as Celtic's midfield enforcer in the team that Martin O'Neill, his old Filbert Street mentor, guided to the Uefa Cup final three years later. But for many, the sight and sound of a stocky, mouthy, red-haired Catholic Ulsterman playing for Celtic was simply too much to bear and enkindled some of the sectarian fires that glow just beneath the surface of civic and cultural life in the west of Scotland.

He was forced to quit international football after threats that were manifest in graffiti painted on the street where he lived in Glasgow and in his home town of Lurgan. "I had played nearly 40 times for Northern Ireland before I came to Glasgow and had no problems," he said in 2010. "But that all changed after I joined Celtic."

He has been physically attacked and abused in the street, and a handful of Rangers fans have been imprisoned as a result. This year, two men were jailed for attempting to send an explosive device to Lennon through the post.

Until recently, though, he has been regularly caricatured in the Scottish press as an immature hothead and a loose cannon who was somehow partly responsible for the vicissitudes he has encountered. One former Scotland international, writing in a Sunday newspaper, even suggested Lennon had brought the death threats upon himself because of his edgy demeanour.

As they flock to praise him now they adopt a supercilious tone in the manner of a headmaster handing over the best-pupil prize to a classroom miscreant who has changed his ways. The patronising tone has not gone unnoticed by Celtic's chief executive, Peter Lawwell, and it irks him. "What people are seeing now is the real Neil Lennon, a man who is highly intelligent, articulate and gifted. We have always known this, though, ever since he started working with us as a player, then captain and now manager.

"Yes, he has changed in the three years he has been manager, but only in the way most people change, including me, three years into any job. There were a few occasions when Neil reacted in a way that he later regretted, but Alex Ferguson, Arsène Wenger and José Mourinho have all reacted similarly in the heat of battle."

When Lawwell speaks about his manager there is barely concealed pride and affection for him. Much of this stems from his achievements as a player and now manager, but also from the way Lennon conducted himself in the face of "events that no other manager in world football has ever had to endure".

Nor will it have escaped the chief executive's notice that his young manager is building a solid reputation beyond Britain. He is now in his third full season as manager of Celtic and club insiders believe he is capable of taking them to heights unimagined since Jock Stein's Lisbon Lions reached two European Cup finals and two semi-finals in the 1960s and 70s.

In Lennon's first season as a Champions League manager Celtic stand poised to qualify for the last 16. Their titanic tactical struggles in the Barcelona double-header have captivated a new audience. Barcelona granted Celtic little more than 20% of possession over their two games, yet this was no frantic anywhere-will-do, rearguard action. Gary Neville, the former Manchester United defender, was impressed. "It wasn't desperate defending; it wasn't legs on the floor, flailing around and diving in. They were controlled, they stayed on their feet. They all did their jobs in a disciplined manner."

They played a bit, too. What also characterised both games against Barcelona was that Celtic had several players who could hurt their lauded opponents. That they did this so effectively was also due to the way Lennon deployed them. He played with two strikers, knowing that his team would have the chance to break and thus secure some dead-ball opportunities. He made them keep a relentlessly narrow shape in the knowledge that Barcelona are uncomfortable playing crosses from wide and that Celtic now have a some intelligent defenders playing together at the same time.

Those who think that Lennon's imminent departure to the Premier League is inevitable may be unable to grasp the strength of the relationships Lennon has forged at Celtic. Last Tuesday, the eve of the Barcelona fixture, 1,000 worshippers crowded into St Mary's church in Glasgow's east end for a mass and celebration to mark the 125th anniversary of Celtic. The club's board and management were joined by seven Barcelona directors who were surprised to learn their slogan, Més que un club, had been coined by a Celtic chairman years earlier in a documentary made to mark their annus mirabilis of 1967, when they became the first British team to win the European Cup.

Earlier that day, as if to underline their commitment to social responsibility, Celtic had announced a gift of £120,000 to four schools to enable pupils to play musical instruments after local authority cuts in musical provision. In the past two years, the club has established initiatives in India and Mexico, where poor communities will receive coaching, football kit and regular visits to Glasgow. The locations were chosen in the knowledge that there will be no commercial spin-off.

Those close to Lennon speak of his appreciation of Celtic's commitment to social justice and the way in which Lawwell and Dermot Desmond, the club's largest shareholder, stood shoulder to shoulder with him when it seemed one strand of Scottish society had unilaterally declared war on him.

He has had to deal with clinical depression and speaks regularly at events to help fellow sufferers. Lennon is also venerated by the Celtic supporters. Barcelona's players lined up to say that the Celtic fans' fervour was unlike anything they had previously encountered, and Lennon's bond with them is the strongest of any of the club's 17 managers.

Some also believe that there would be an incongruity in Lennon managing an English club that is the mere plaything of foreign plutocrats; where success can be bought rather than earned and where the desire for instant gratification has rendered the values of sacrifice and industry utterly redundant.

Lennon's entire career has been lived in the shadow of adversity; it is as if he is destined always to scale his heights by the north face in winter while others are granted a gentler route. He may be at Celtic for some time yet.